I'm making a test setup for MS recording.
The 1st setup is with a figure 8 mic and a cardioid. The 2nd setup is with a figure 8 mic and a omni. The 3rd setup would be with 3 cardioids. (2 S channel placed exactly(!) opposite and phase aligned)
I found that by panning the two S cardioid mics hard left and right and adding M did not result in the same audio perception as with a figure 8 and cardioid.
What I did to resolve this: First summing the two mics to make a new figure 8 signal. 1st S channel phase non inverted -> bus 1 2nd S channel phase inverted > bus 1
Then treat bus 1 (the 'new' figure 8 signal) as you would do normally with S signals bus 1 > new channel 3 and pan hard left, phase non inverted bus 1 > new channel 4 and pan hard right, phase inverted channel 5 has the M signal.
This results in a much better MS with 3 cardioids.